Method of navigating an automated guided vehicle

ABSTRACT

Method of navigating a vehicle (11) moving generally in one plane in which the actual position and direction (L) of the vehicle is determined by measuring the angles in said plane from a reference point of the vehicle (X, Y) to fixed points in relation to a reference direction on the vehicle. A plurality of points (X 1  Y 1 , X 2  Y 2 , X 3  Y 3 ) anonymous but fixed as to the position thereof are arranged for the navigation, and the position of all the fixed points together with existing sight obstacles between the reference point of the vehicle and the fixed points are registered in an electronic memory. The movement of the vehicle in said plane is initiated in a known position and with a known direction for identifying the fixed points visible from the reference point in said position. In each position and direction of the vehicle during movement thereof the latest known position and direction are utilized in determining the identify of the fixed points visible at that moment.

The invention is related to a method of navigating an automated guidedvehicle.

In industry there is an increased use of automated guided vehicles intransporting goods and material between different stations in storingpremises and workshops. Then, the trucks are controlled automaticallyalong predetermined paths between the various stations. In a reliablemethod of the truck steering the truck control system detects a cableloop arranged within or on the floor, or a painted loop on the floor.Such a control system is very reliable and simple to use but theconditions in the premises where the trucks are being used, are notalways such that it is suitable to use a control loop within or on thefloor, and in addition to this the system suffers from the no littledrawback that the truck movement is limited strictly to the pathsdefined by the control loop, and that these paths cannot easily bealtered because they are permanent installations.

Accordingly, systems of navigating automated guided trucks have beendeveloped wherein the truck movement is not restricted to a permanentlyarranged control loop, the truck being provided with apparatuscontinuously defining its current position in relation to fixed objectsin the premises through which the truck is moving. The present inventionis related to this type of navigation system and accordingly concerns amethod of navigating a vehicle moving generally in one plane in which acalculation of the current position and direction of the vehicle iseffected by measuring the angles in said plane from a reference point onthe vehicle to fixed positions in relation to a reference direction onthe vehicle.

In a prior art systems for free navigation of automated guided vehiclesa laser beam is used detecting reflectors fixedly arranged in thepremises where the truck is moving. This system is called TURTLE and isdescribed in the FMS Magazine, July 1983, pp 232-236. It is based on thefact that the truck for each position to be determined "sees" a numberof fixedly arranged reflectors in the premises and that these reflectorscan be identified, which is effected by the optical signal emitted fromeach of the reflectors, being coded. This is done by the laser beamemitted from the truck scanning over 360° and, when passing a reflector,generating a bar coded reflected signal.

Another prior art system described in Journal of Dynamic Systems,Measurement, and Control Vol. 105, September 1983, pp 152-155 is basedon a type of position calculation similar to the TURTLE-system, butusing instead of laser light ultra sound for locating and identifyingthe fixed positions in the space. From each fixed position a soundsignal is then emitted having a predetermined frequency unique to theactual position. The latter system is similar to the system described inRobotics Age, March/April 1983, pp 31-33 and which is also subjectmatter of U.S. Pat. No. 4,328,545. However, in that case beacons areprovided in the fixed positions for emitting light individually codedsuch that a receiver on the truck can identify the beacon emitting thelight signal.

As far as the inventor of the present invention knows, prior art systemsfor free navigation of automated guided trucks are based on the factthat the signals received by the truck to be used for calculation of thecurrent position of the truck, are specific to the points from whichthey originate, and can be used to identify these points. This is alimitation of the prior art systems, and the object of the invention isto make possible the use of simple and inexpensive reflectors providingthe signals from the fixed points in the space without the necessity ofcoding these signals for identifying purposes. At the same time it willbe possible to determine the position even if some of the fixed pointsare temporarily hidden or there are reflectors not associated with thesystem.

For this object the method according to the invention has been given thecharacteristics appearing from claim 1.

In order to explain the invention in more detail reference is made tothe accompanying drawings, in which:

FIG. 1 is a plan view of premises through which an automated guidedtruck is to navigate by applying the method according to the invention,

FIG. 2 is a side view of the truck,

FIG. 3is a horizontal sectional view of the truck in FIG. 2 taken alongline III--III,

FIG. 4 is diagrammaticside view of the optical system for emitting ascanning laser beam,

FIG. 5 is a geometrical figure for calculating the position anddirection of the vehicle from the angles to three fixed referencepoints,

FIG. 6 is a block diagram of an electronic system for working the methodaccording to the invention,

FIG. 7 is a more detailed block diagram of the electronic systemarranged on the truck,

FIG. 8 is a diagrammatic front view of a detector suitable for use inthe system,

FIG. 9 is a diagrammatic plan view illustrating the scanning of areflector by use of the detector in FIG. 8, and

FIG. 10 is a graph illustrating the signal obtained from the detectorfrom the scanning according to FIG. 9.

With reference to FIG. 1 a possible path 10 is shown to be followed byan automated guided truck 11 by applying the method according to theinvention. The path extends through a free space in the premises,limited by the walls of the premises and/or objects existing therein,such as machines, storage racks or the like, here generally indicatedwith 12. The free space through which the truck can move does notactually have to be limited by fixed objects or be enclosed by walls;the space for the truck movement may be limited to a certain area havinga specific shape and extension in the horizontal plane for otherreasons. When utilizing the method according to the invention aplurality of reflectors 13 shall be mounted in the motion space, thereflectors being of the most simple and inexpensive type and aresuitably made of reflecting tape, for instance of the type marketed by3M Svenska AB under trademark SCOTCH LITE. The tape may be attacheddirectly to walls or objects or may be mounted on located verticallystanding pins and having a diameter of about 20 mm. The tape should havesome extension in the vertical direction. Behind the reflectors a darkbackground should be arranged to make the reflectors sharply outlinedagainst the background.

On the truck an optical system 14 is arranged for emitting a laser beamscanning 360°. The truck is constructed in a special way for themounting of the optical system, FIG. 2 and FIG. 3. Thus, it comprises achassis 15 supported on two uprights or columns 16 and two flanges orwebs 17 the body 18 of the truck, leaving a gap 19 between the chassisand the body. In this gap the optical system 14 is mounted in such a waythat the flanges or webs 17 extend radially to the vertical scanmovement axis 20. In that way there is an almost unbroken area for thescanning laser beam around the axis 20 at a level close to the floor orground. Though the beam per se is harmless, a lot of people are of theopinion that every laser beam, though being weak, is dangerous to theeyes. In the described arrangement the beam is far below eye-level, thisplacing of the optical system providing also the advantage that the bodyand the load carried thereon at no occasion screens the laser beamduring the scanning movement thereof, which is not the least importantfor a secure and reliable navigation.

The body 18 of the truck may comprise suitable lifting means or otherload handling means and space for electronic equipment, batteries etc.

The optical system may in principle be arranged as is diagrammaticallyshown in FIG. 4. A laser generator 21 which may be a gas laser (He-Ne)for visible light or a semiconductor laser for IR-radiation, possiblypulsed, directs its beam towards a mirror 22 which in turn directs thebeam through an opening 23 in a mirror 24 to a mirror 25. The lattermirror is supported on a rotatably mounted balance wheel 26 connectedover a drive belt to a drive motor 27 to be rotated at a constantrotational speed of the order of 2 or 3 revolutions per sec. The balancewheel is connected to an angle meter 28 of very high accuracy, or to adevice for measuring the period used by the balance wheel to rotate at aconstant speed from a reference position to a given position, the angleposition of which is to be indicated. Such time measurement can beperformed at very high reliability and by utilizing a simpler and lessexpensive device than an angle sensor of high accuracy.

When the laser beam hits a reflector 13 it is reflected back to themirror 25 transmitting the reflected beam onto the mirror 24 from wherethe reflected beam by suitable optics 29 is directed onto a detector 30to be focused thereon.

In FIG. 5 the truck is shown in a co-ordinate system, and in order toidentify the position of the truck in a given moment the co-ordinates xand y for the axis 20 and the angle φ have to be determined. The angle φis the angle between the x-axis and a vehicle reference line L, usuallythe longitudinal axis of the vehicle. This determination can be done byutilizing three fixed points, i.e. by using three reflectors, which thetruck "sees" from the actual position. On the basis of the angles φ₁, φ₂and φ₃ between the sight lines from the reference point 20 of the truckto the three reflectors having the co-ordinates (x₁ y₁), (x₂ y₂) and (x₃y₃), and the reference line L, the position and direction of the truckcan be determined by applying conventional trigonometry as previouslyknown per se and as described in the above mentioned publication Journalof Dynamic Systems, Measurement, and Control, September 1983, Vol.105/153. Thus, by utilization of the optical system on the truck theangles φ₁, φ₂, φ₃ are measured and on the basis of these angles theequation is determined for those two circles which in FIG. 5 have beendrawn through the three points and the position of the truck forcalculating the position and direction of the truck. Since thistechnique is previously known per se these trigonometrical calculationsare not presented here. There is, however, a substantial differencebetween the system described here and those systems previously known anddescribed in the references initially referred to: the three pointsneeded for the calculations have not identified themselves by a coding.The truck can see more fixed points than three and, thus, it isnecessary to identify these points in one way or the other.

The motion area with reflectors and path are stored in an electronicmemory. The storing in the electronic memory can be done by using knowntechnique. A method which can be applied is the utilization of a CADsystem, for instance AutoCAD, which is a designing program by means ofwhich any drawing can be reproduced in a microcomputer. By using programthe plan of the truck movement area of FIG. 1 can be programmed into amicro computer. To store the decided truck path the truck as analternative to storing via the AutoCAD program can be driven along saidpath and its position in relation to the reflectors be continuouslystored in the program. The positions of the reflectors are determined byutilization of conventional land surveying technique, and the measuredpositions are stored in the program. As an alternative the reflectorpositions can be located by driving the truck along a known path and bedetermined by utilizing the ptical system and a program loaded in thecomputer system of the truck.

Then, when the truck shall automatically follow the defined path, atruck position and direction must first be defined so as to serve lateras a base for identifying subsequent positions and directions. This canbe done in different ways. According to one method the truck is placedin a predetermined position and in a predetermined direction initiallydefined. According to another method the position and direction aredefined from three fixed points at the time, assuming that said pointshave certain identities, and this is repeated with reference to allfixed points visible from the truck in a given position. This results ina plurality of positions and directions for each three points, whichform the basis for the calculation. The variance of the obtained resultis calculated, and then the calculations are repeated with anotherassumption as to the identity of the points. When the assumption isfound, which giving the least variance, the points are identified.

As the truckis moving, consecutive calculations of the truck positionand direction are made in each individual position on the basis of theangle measurements made by the apparatus 14 to the reflectors visiblefrom the truck in the actual position by guidance of the obstaclesprogrammed. Thus every new positioning of the truck will be based oninformation previously obtained with regard to the truck position andmoving direction.

It should be noted that in each truck position several determinations ofthe truck position and direction may be made depending on how manyreflectors reaction may be made depending on how many reflectors thatare seen by the truck from the actual position. since the reflectors areof a simple and inexpensive nature and do not have to transmit a codedreflected signal, the detectors can be set out generously whereby a highaccuracy in the calculation of position and direction can be obtained.

To work the method according to the invention by using the abovementioned calculations a computer system constructed in principleaccording to FIG. 6 is used. A controlling stationary computer 31 whichcan be common to many trucks, is provided for programming the motionarea and the desired paths in this area, for instance by using the abovementioned AutoCAD program. The computer 31 is also connected to aoperator's console 32 with a keyboard to allow temporary manualoperations in the program loaded. The computer 31 is connected through acommunication system 34, FIG. 6, for instance a wireless link (radio orIR) to a micro computer 35 on each truck.

The computer 31 operates as a coordinator of several trucks in a systemand gives through the link the necessary information to the computer 35on the truck. The computer 35 can be a micro computer of the typeMotorola MC 68010, and it is connected to another micro computer 36 onthe truck which also can be of the type Motorola MC 68010 receivingangle measurement signals from the optical system 14 and processingthese signals so as to give to the computor 35 information on themomentary position and direction of the truck by applying the abovementioned method. Then in the computer 35 a comparison with theprogrammed data is made, and depending on the result of the comparisoncontrol signals are sent to the driving system 37 of the truck toprovide the correction of the truck movement necessary to make the truckmove along the predetermined path. Also connected to the computer 35 isan appropriate sensor system 38 to stop the truck movement when a riskof collision with persons or objects arises.

The accuracy in the control of the truck can be substantially improvedby utilizing a detector of the type described in FIG. 8. This detectoris of a known design and comprises two silicon photodiodes indicated at39A and 39B. Each of the diodes has a width of the order of 100 μm, andthe diodes are separated by a gap 40 of the order of 10 μm. The scanningof the reflectors 13 in principle can be regarded as a projection of thedetector, that is the two silicon diodes, on the surface of thereflector. This is illustrated in FIG. 9. When the projected image ofthe detector is entering the reflector in the direction of the arrow 41,one silicon diode 39A will first be in the area of the reflector andthen the other one. By adding the signals from the silicon diodes withreversed polarity the detector will provide a signal as illustrated inFIG. 10 where the horizontal axis defines the scanning angle and thevertical axis defines the signal from the detector. When the image ofthe diode 39A only is projected on the reflector, the signal from thedetector will rise to a largest positive value so as to decrease againto zero later when the second diode starts to enter the reflector, asthe signal provided by the second diode is subtracted from the signalfrom the first diode. after that no signal from the detector will beobtained until the projection of said first diode moves from the surfaceof the reflector, the signal from said second diode being emitted fromthe detector. In other words, by this arrangement there is obtained adistinct signal easy to define, which means that the determination ofthe angle can be done at high accuracy with a corresponding highaccuracy in the determination of the position and direction of thetruck.

The determination of the position and direction by applying the abovedescribed method can be completed with odometry, which means thatsensors on the wheels and steering mechanism of the truck providesignals representing the distance run and the direction of the existingmovement, respectively, and that these signals then are used forcalculating the change of the position and direction of the truck, tothat it is possible to obtain, on the basis thereof by calculationsexecuted in the computer 36, information on the actual position anddirection of the truck. The quantities hereby obtained then may beprocessed together with those obtained by applying the method accordingto the invention.

By using quantities representing the speed of the truck and easilycalculated by known methods, it is also possible to compensate for thetruck movement between the measurements of the angles to the fixedpoints. In other words, the truck is not immobilized during themeasurements, and even if there is a slight movement it does effect thecalculations of the position.

Means can be provided for manually controlling the truck, for instance acontrol panel connected to the truck through a cable.

Some minor lateral deviation between the path followed by the truck onone occasion and the path followed in the same part of the path onanother occasion can be programmed providing an advantage as the truckin that case does not wear wheel tracks in the support (the floor)during movement along an unchanged path for a long time.

Essential of the invention is the method in which the position anddirection of movement of the truck is determined by utilizing a numberof reflectors providing anonymous reflection signals. The practicalembodiment of the mechanical and electronical means through which thismethod is realized has been illustrated and described in the embodimentpreferred at present but can be modified or replaced by other means thanthe one described here without departing from the scope of the appendedclaims.

I claim:
 1. Method of navigating a vehicle for instance an industrialtruck moving generally in one plane by which a determination of theactual position and direction of the vehicle is provided by measuringthe angles in said plane from a reference point on the vehicle to fixedpoints in relation to a reference direction of the vehicle at leastthree angles being used for a trigonometric determination by calculationmeans of position and direction, characterized in that a plurality offixed points anonymous but determined as to the position thereof arearranged for the navigation, that the position for each of the points aswell as existing sight obstacles between the reference point of thevehicle and the fixed points is stored in an electronic memory, that themovement of the vehicle in said plane is initiated in a known positionand with a known direction so as to identify the fixed points visiblefrom the reference point in said position, and that in each position anddirection of the vehicle during the movement thereof, the latest knownposition and direction are utilized to determine with the aid of thestored position of the sight obstacles the identity of the fixed pointsvisible at the moment.
 2. Method according to claim 1, characterized inthat the bearing of the fixed points are taken by emitting a light beamfrom the vehicle, scanning horizontally, and by reflection of the lightbeam from reflectors in the fixed points and reception of the reflectedlight beam on the vehicle.
 3. Method according to claim 2, characterizedin that the storing of the fixed points is effected by reproducing saidplane with sight obstacles and the fixed points in said electronicmemory.
 4. Method according to claim 3, characterized in that thereproducing is effected in a CAD system for visual representation ofsaid plane with sight obstacles and the fixed points, the path to befollowed by the vehicle being included in the visual representation. 5.Method according to claim 2, characterized in that the determination ofthe position of the fixed points is effected by driving the vehiclealong a known path while measuring the angles to the fixed points byutilizing the optical system and the program controlled computing andstoring of the positions of the fixed points in the electronic memory.6. Method according to claim 2, characterized in that the angles aredetermined by measuring the period for the movement of the light beamscanning horizontally at constant speed, from a reference point to aposition where it hits a reflector.
 7. Method according to claim 2,characterized in that the reflected light beam is detected by utilizingtwo photo detectors arranged side by side, the outputs of which areadded with reversed polarity and which are hit, one after the other, bythe reflected light beam.
 8. Method according to claim 2, characterizedin that the light beam is emitted close to the plane of the floor orground.
 9. Method according to claim 1, characterized in that thevehicle during movement thereof is brought to a position in relation toa desired path, which is displaced more or less towards one side or theother of the nominal path on successive tours along said path.
 10. Truckfor carrying out the method according to claim 1 comprising a chassis(15) and a body (18), characterized in that the body (18) is supportedon the chassis (15) at a limited number of positions leaving a gap (19)between the body and the chassis, and that means (14) for emitting alight beam scanning horizontally is placed in the gap, existing means(16, 17) for supporting the body on the chassis of said positions havinga limited extension in the path of the scanning movement of the lightbeam.